More than four years have passed since the outbreak of the severe acute respiratory syndrome (SARS) epidemic, and still very little is known about the molecular biology and pathogenesis of this deadly virus. Among the accessory proteins of the SARS coronavirus (SARS-CoV), the 3a protein has been shown to interact with the spike, envelope, and membrane glycoprotein and has recently been established to be a structural component of capsid. Recent studies suggest that the 3a protein may function as an ion channel and may promote virus release. In order to further characterize the functional properties of this protein, we initiated studies to check its RNA binding activity. Using the yeast three-hybrid system, electrophoretic mobility shift assay (EMSA), and ultraviolet (UV) cross-linking techniques, we have shown that the 3a protein is capable of binding specifically to the 5' untranslated region (5'UTR) of the SARS virus genomic RNA. Further, we have mapped the interaction domain of the 3a protein responsible for this RNA-protein interaction using a series of deletion mutants and defined it to the central 75 amino acid region. This RNA binding motif of 3a does not share homology with any other known RNA binding protein and may have an important role in viral capsid assembly and pathogenesis.